US2373257A - Process for precipitating alkali metal hydroxide - Google Patents

Description

' 2,373,257 rich PROCESS FOR- PRECIPITATING 1'1. 11 METAL mznnomn Irving a. Muskat, Akron, Ohio, assignor to rats. burgh Plate Glass Company, Pittsburgh, Ba, a corporation of Pennsylvania No Drawing. Application March 4, 1942,

erial No. 433,359

Claims.

This invention relates to a method of concen trating an aqueous alkali metal hydroxide and is particularly related to the production of conknown that concentrated sodium hydroxide could be secured by cooling 9. 'more dilute solution in order to crystallize out of solution a hydrate which is more concentrated as to sodium hydroxide and recovering the hydrate. This process ordinarily requires extensive cooling which is expensive. Moreover, solutions of sodium hydroxide are extremely viscous at temperatures at which the hydrate thereof normally crystallizes, and in consequence, crystals precipitating therefrom remain suspended therein and do not settle out of solution. Due to the high viscosity of such solutions, a recovery of crystals .is extremely difiicult and thus, the concentration of caustic by crystallization of hydrates thereof has not achieved success.

In United States Patent No. 1,961,590, granted to MacMullin, it is stated that sodium hydroxide monohydrate can be precipitated from aqueous solutions containing upwards of percent of sodium hydroxide by treatment of the solution with ammonia. However, this patent points out that this process is objectionable because of the amount of ammonia required.

In accordance with my invention, I have found that by treatment of aqueous solutions of alkali metal hydroxide preferably containing at least 40 percent, preferably not less than about percent sodium hydroxide, only small amounts of ammonia are required to cause precipitation of a solid hydrate and that it is possible to, precipitate hydrated hydroxide which is substantially higher in concentration (generally containing in excess of percent and often about percent by weight of sodium hydroxide) than the solution undergoing treatment. By treating such solutions and recovering the solid hydroxide which is precipitated, concentrated sodium hydroxide may be recovered in a simple manner with only a small quantity of ammonia and without contamination of the caustic. By use Of a larger quantity of ammonia, a product substantially higher in concentration containing to 100 percent NaOH may be obtained.

The process may be regulated by controlling the temperature and the amount of ammonia present in the system. At a temperature of-about 25 0.,

a small amountof ammonia added to a solution of sodium hydroxide containing 40 to 50 percent or more of NaOI-l causes precipitation of solid hydroxide which when separated. from mother liquor possesses a concentration of 60 to (u per-v cent sodium hydroxide. By adding further amounts of ammonia, further quantitiesof caus tic are precipitated and the caustic may be more concentrated. Upon treatment of such a solution at a higher temperature, for example, 35 to 60? C., or above, with ammonia, the mixture separates into two liquor layers, the lower of which contains a relatively high concentration of sodium hydroxide, a portion of water and a relatively small amount of ammonia, while the upper layer contains the major portion of the ammonia, a portion of the water, and a minor quantity of caustic. Upon addition of a still further quantity of ammonia, a solid hydroxide precipitates in the presence of the two liquid layers. When a further quantity of ammonia is added, the two separate liquid phases cease to exist and the solid hydroxide remains in equilibrium with a single liquid phase. This hydroxide when recovered contains 69 percent or often in excess of.85 to percent of sodium hydroxide.

Thus, when about 0.1 to 1.1 parts of ammonia are mixed with one part of a 50 percent solution of sodium hydroxide in a suitable closed container at 60 0., two liquid phases are formed which separate in the container as an upper and lower layer. Upon adding more ammonia, however, the

two liquid phases merge and solid caustic exists in equilibrium with a liquid phase containing ammonia, water, and caustic. This solid is of substantially high concentration, generally having a concentration of at least 69 percent or above, and if sumcient ammonia is present, the solid may be substantially anhydrous caustic.

There is a marked difierence between the amount of ammonia required to cause separation of a pair of liquid phases or to cause precipitation of solid hydroxide from a solution containing 50 percent or more of sodium hydroxide and that required to cause precipitation from a solution containing onlLv 20 to 25 percent sodium hydroxide.

The following table illustrates the differences in the amounts, at a temperature of 25 to 30 C.

Grams of $133 35. NHweg Cone. of NaOH soln., per cent m of N l l in solution to 1 ti emasst;

imam tallization for most purposes,-up to about two or three partsof ammonia is used per part of sodium hydroxide in solution.

In accordance with the present invention, I have found that caustic may be efiectively concentrated by first treating the caustic with sufflcient ammonia to cause separation of two liquid phases, causing the two phases to merge and precipitating solid sodium or other alkali metal hydroxide. These phases may be caused to merge by addition of further ammonia or by cooling I the mixture to a suitable temperature, for example, below 35 C. to cause the liquid phases to 'merge and formation of solid hydroxide of high concentration in' equilibrium with a single liquid phase. This method is particularly desirable, involving the step of first forming two liquid phases followed by the step of causing the two liquid phases to merge, either by cooling or by addition of ammonia, in treating solutions of 40 or 50 percent since in treating solutions of such high concentration with ammonia, it is found that if a large amount of ammonia is added initially, caustic often precipitates in large lumps which when removed contain so much mother liquor that little or no concentration is secured. In accordance with the present method, it is possible to eliminate this difllculty.

The process may be conducted batchwise or continuously in any suitable type of closed apparatus. Suitable apparatus and methods of treatment have been described in my application Serial No. 179,334, filed December- 11, 1937, now

Patent No. 2,196,594. When substantial amounts of sodium chloride are present in the solution, a purified hydroxide is prepared by the present process. This method has been described and claimed in my application SerialNO. 179,336,

filed December 11, 1937. The present invention,

however, is directed mainly to the concentration of sodium hydroxide in various solutions, including solutions which may contain little or no sodium chloride.

The following examples are illustrative:

Example I 0.15 part by weight of anhydrous liquid ammonia was mixed with one part by weight of a solution containing percent sodium hydroxide at a temperature of 45 C. The mixture was then cooled to 25 C. whereupon solid hydroxide precipitated and the two liquid phases merged. Upon recovery of the solid it was found to contain 68 percent NaOH. i

Example I! was recovered by filtration and was found to contain about 90 per cent sodium hydroxide.

Although the process is particularly adapted to the use of anhydrous liquid ammonia, mixtures of water and ammonia which contain suiiicient ammonia (in general about NHJ) to cause precipitation of solid hydroxide may be used, if desired. While this process has been described for the most part with relation to sodium hydroxide and its hydrates, the process is equally applicable to the preparation of any of the hydrates of other alkali metal hydroxide, such as those of lithium or potassium hydroxide.

Although the present invention has been described in connection with the specific details of certain embodiments thereof, it is not intended that such details shall be regarded as limitations upon the scope of the invention, except insofar as included in the accompanying claims.

This application is a continuation-in-part of application Serial No. 325,789, filed March 25, now Patent No. 2,285,300, issued June 2,

I claim:

1. A method which comprises treating aqueous sodium hydroxide with a material .of the group consisting of liquid ammonia and mixtures of liquid ammonia and water, the amount of ammonia being suificient to insure the existence of a liquid phase containing a major portion of the ammonia and a second. liquid phase containing a substantial portion of the hydroxide, adding more ammonia in an amount sufilcient to cause the two liquid phases to merge and separating solid hydroxide which is precipitated in the mixture from the ammonia liquor.

2. A method which comprises treating aqueous alkali metal hydroxide with a. material of the group consisting of liquid ammonia and mixtures of liquid ammonia and water, the amount of ammonia being suiiicient to insure the existence of a liquid phase containing a major portion of the ammonia and a second liquid phase containing a substantial portion of the hydroxide, adding more ammonia in an amount suilicient to cause the two liquid phases to merge and separating solid hydroxide which is precipitated in the mixture from the ammonia liquor.

3. A method of concentrating aqueous alkali metal hydroxide which comprises treating said hydroxide with a material of the group consisting of liquid ammonia and mixtures of liquid ammonia and water, the amount of ammonia being suflicient to insure the existence of a liquid phase containing a major portion of the ammonia and a second liquid phase containing a substantial portion of the hydroxide and cooling the mixture to a temperature at which the liquid phases merge and solid hydroxide precipitates and separating the solid from the ammonia.

4. The process of claim 3 wherein the solution contains at least 40 percent NaOH.

5. A method which comprises treating aqueous alkali metal hydroxide with a material of the group consisting of liquid ammonia and mixtures of liquid ammonia and water, the amount of ammonia being sufiicient to cause the formation of a ,liquid phase containing a major portion of the ammonia and a second liquid phase containing a substantial portion of the alkali hydroxide, adding more ammonia while simultaneously cooling until the two phases merge into a single liquid phase, and recovering the solid alkali hydroxide which is precipitated.

IRVING E. MUSKAT.